The explosions involving layered fuel-air mixtures are of interest both for safety and technical applications. This paper deals with the analysis of flame propagation in large-scale layered mixtures, aiming at the prediction of consequences of explosions when flammable clouds are formed from accidental liquid spills or gas leaks in closed or partially confined environment [1-8]. Infact, following the initial dispersion of lighter than air gases or vapors, it is very likely that a layered mixture is formed, having a large portion rich or lean outside the flammability limits. Then only a fraction of the total mixture released is typically involved in the explosion. A common over-conservative assumption instead is that the whole amount of the escaped flammable gas forms an homogenous stoichiometric mixture and particpates in the explosion. To this regard the mitigation devices design is more expensive. The flame propagation process in layered mixtures, experimentally observed firstly by Phillips [1], is characterised by the contemporary presence of three modes of combustion. A laminar premixed combustion front travels across the layer of fuel-air mixture initially within the upper (UFL) and lower (LFL) flammability limits, strictly followed by a diffusive combustion front, which forms in correspondence of the stoichiometric line, between the excess of fuel in the rich layer and the excess of air in the poor layer. Finally, if a rich layer is initially present, a convective flame is established, after a certain distance, due to the convective mixing of the unburnt fuel with the air determined by the hot products of the previous combustion modes. So far, few models for this type of flame propagation have been developed and presented in the literature [2-5,8]. For the engineering purposes there is the need to reproduce this complex phenomenon, considering furthermore real environments and not a small laboratory scale domain. This is a formidable task due to the very large scale disparity: in space between the laminar premixed front thickness and the typical dimension of the devices involved, in time between the chemical reaction rate or the pressure wave velocity and the time of flame propagation across the domain. A CFD approach to reproduce the laminar flame propagation in layered methane-air mixtures, adopting a grid size suitable for evaluating large-scale gas explosions, is proposed by implementing a new combustion model as user subroutine of a commercial solver named CFD-ACE+, developed by CFD Research Corporation, USA [9]. At the moment, only the first premixed laminar combustion model has been implemented. The results are validated over experimental test cases where the layered fuel air mixture allows for the presence of this mechanism alone [3].